The observation of C-H bending, and C-C and C-H stretching vibrational levels in high resolution C 1s−1 π* photoabsorption spectra of ethylene and benzene demonstrates that both molecules have broken symmetry in this core excited state. A simple model based on the antibonding character and the out-of-plane charge distribution of the π* electron is utilized to explain our observations and to show that both core excited molecules become non-planar. The model is also consistent with the vibrational structure associated with the transitions of C 1s → Rydberg states in ethylene, which is found to be very different from that of the C 1s−1 π* state.
